In the development of radio communication systems, such as mobile communication systems (like for example GSM (Global System for Mobile Communication), GPRS (General Packet Radio Service), UMTS (Universal Mobile Telecommunication System) or the like), efforts are made for an evolution of the radio access part thereof. In this regard, the evolution of radio access networks (like for example the GSM EDGE radio access network (GERAN) and the Universal Terrestrial Radio Access Network (UTRAN) or the like) is currently addressed. Such improved radio access networks are sometimes denoted as evolved radio access networks (like for example the Evolved Universal Terrestrial Radio Access Network (E-UTRAN)) or as being part of a long-term evolution (LTE) or LTE-Advanced. Although such denominations primarily stem from 3GPP (Third Generation Partnership Project) terminology, the usage thereof hereinafter does not limit the respective description to 3GPP technology, but generally refers to any kind of radio access evolution irrespective of the underlying system architecture. Another example for an applicable broadband access system may for example be IEEE 802.16 also known as WiMAX (Worldwide Interoperability for Microwave Access).
In the following, for the sake of intelligibility, LTE (Long-Term Evolution according to 3GPP terminology) or LTE-Advanced is taken as a non-limiting example for a broadband radio access network being applicable in the context of the present invention and its embodiments.
However, it is to be noted that any kind of radio access network may likewise be applicable, as long as it exhibits comparable features and characteristics as described hereinafter.
In the development of cellular systems in general, and access networks in particular, relaying has been proposed as one concept. In relaying, a user equipment or terminal (UE) is not directly connected with an access node such as a radio base station (e.g. denoted as eNodeB or eNB) of a radio access network (RAN), but via a relay node (RN). Relaying by way of relay nodes RNs has been proposed as a concept for coverage extension in cellular systems. Apart from this main goal of coverage extension, introducing relay concepts can also help in providing high-bit-rate coverage in high shadowing environments, reducing the average radio-transmission power at the a user equipment (thereby leading to long battery life), enhancing cell capacity and effective throughput, (e.g. increasing cell-edge capacity and balancing cell load), and enhancing overall performance and deployment cost of radio access networks.
FIG. 1 shows a schematic diagram of a typical deployment scenario of a relay-enhanced access network, such as e.g. a Long Term Evolution (LTE) RAN with radio-relayed extensions. As shown in FIG. 1, UEs at disadvantages positions such as a cell edge and/or high shadowing areas are connected to a so-called donor base station (DeNB) via a respective RN. The link between DeNB and RN may be referred to as backhaul link, relay link or Un link, and the link between RN and UE may be referred to as access link or Uu link.
A UE Evolved Packet System (EPS) bearer may be considered as a virtual connection between a core network (CN) and the UE, which is characterized by different quality of service (QoS) parameters, and as such the traffic belonging to this bearer will be treated according to these parameters on the different nodes between the gateways and the UE. On the other hand, RN bearers, also referred to as Un bearers, are defined between the RN and DeNB. The mapping of UE EPS bearers and RN bearers can be done either one-to-one (where there is one Un bearer for each UE EPS bearer), or many-to-one (where several UE EPS bearers are mapped into one Un bearer). The many-to-one mapping can be based on mapping criteria such as the QoS requirements or can be done on a per UE basis (i.e. one Un bearer for all bearers of a given UE, regardless of QoS).
In the context of LTE and LTE-Advanced, a Layer 3 (L3) RN, also referred to as Type I RN, is currently taken as a baseline case for the study on relay extensions. Currently, four options for candidate relay architectures are conceivable, the details thereof being out of scope of the present invention. The four candidate relay architectures may be grouped into two categories.
In a relay architecture of a first category, the DeNB is not aware of the individual UE EPS bearers. That is, the relayed UEs are hidden from the DeNB, and the DeNB is aware of only the RNs with which the relayed UEs are connected. Thus, in such a relay architecture only many-to-one mapping is supported, and specifically QoS based mapping (assuming the QoS mapping is done in a node before the DeNB through a marking of the IP headers Type of Service (TOS) field, for example, in accordance with the a QoS parameter such as Quality of Service class identifier (QCI)).
In a relay architecture of a second category, the DeNB is aware of the individual UE EPS bearers of all of the relayed UEs. That is, the DeNB is aware of the relayed UEs as well as of the RNs with which the relayed UEs are connected. Thus, in such relay architecture, it is possible to support both many-to-one (including per UE based mapping) and one-to-one mapping, and the mapping can be done at the DeNB itself, as the UE EPS bearer's information is visible at the DeNB. Even if many-to-one mapping is used, a more appropriate mapping can be employed in the second category architecture as compared with the first category because all the QoS parameters (in addition to the QCI) can be used in the mapping process.
The split of resources between the DeNB-RN link and the RN-UE link may be done dynamically or semi-dynamically depending on the number of UEs connected to the DeNB and to the RNs. In the following, centralized resource partitioning assumed, where the DeNB assigns the resources that each RN connected to it can use to serve its connected UEs. The user scheduling is done at the RNs assuming only the resources assigned by the DeNB are available. Yet, it is noted that distributed resource partitioning may be equally used as well.
In the context of LTE and LTE-Advanced (i.e. in the context of release 8 specifications), a so-called X2 interface is specified as an interface for the interconnection of radio base stations, i.e. two or more eNBs, which may be provided by different vendors, within a radio access network such as an E-UTRAN. The X2 interface is to support an exchange of signaling information as well as user data, and is a point-to-point logical interface being feasible even in the absence of a physical connection between the corresponding eNBs.
The main purposes of information exchanges on the X2 interface relate to UE mobility, load balancing, and inter-cell interference coordination.
Regarding UE mobility, the X2 interface is defined as the default interface for UE mobility. This accelerates the overall handover process by decreasing the time taken during handover preparation as well as the data forwarding time as the source and target eNBs are communicating directly via the X2 interface without involving the core network (CN)
Regarding load balancing (LB), it is noted that there is no centralized radio resource management (RRM) functionality in LTE and LTE-Advanced and the RRM is performed in a decentralized fashion. Hence, there is a need for communicating load information between neighboring eNBs so that a potential imbalance between loads of the eNBs can be counter-balanced. For example, handover threshold parameters may be increased to prevent many UEs from being handed over to an already overloaded eNB. The load balancing information in LTE and LTE-Advanced is sent via the X2 interface.
Regarding inter-cell interference coordination (ICIC), apart from the load information, it is beneficial to eNBs to be aware of the resource utilization in neighboring cells, as LTE and LTE-Advanced uses a reuse factor of 1. For ICIC operation in the downlink, a relative narrowband transmit power (RNTP) bitmap is communicated between the eNBs, telling their neighbors the relative power they are planning to transmit for each resource block (RB). From the RNTP bitmap from all neighboring eNBs, an eNB for example can decide not to schedule cell edge users on specific RBs that most of the neighbors are planning to transmit on. In the uplink, an overload indicator (OI) and high interference indicator (HII) messages are used to facilitate ICIC. The OI summarizes the average uplink interference and noise for each RB, and neighboring eNBs can communicate the OI between themselves via the X2 interface and use it for optimal uplink scheduling. As compared with OI, which is a reactive measure based on information on past transmissions, the HII is a pro-active measure that indicates that the eNB is planning to use certain RBs for cell-edge UEs in the near future. The HII is communicated between neighboring eNBs via the X2 interface and it can be used to prevent a situation where cell-edge UEs belonging to neighboring eNBs being scheduled to use the same RBs at the same time, leading to low uplink signal-to-interference-plus-noise ratio, and hence low uplink throughput.
In addition to the above-outlined usage of the X2 interface in the context of LTE and LTE-Advanced (without relay extensions), the X2 interface is currently also proposed to be used in the context of relaying and relay-enhanced LTE and LTE-Advanced environments. In this context, the X2 interface is specified between a relay node (RN) and its associated donor base station (DeNB), its neighboring relay nodes (RNs), and non-donor base stations (eNBs) (herein, non-donor eNBs refer to eNBs that are not the controlling or donor eNB for the concerned RN, while these eNBs can be DeNBs for other RNs) within a radio access network such as an E-UTRAN.
The X2 interface in relaying contexts may be specifically beneficial for handover scenarios. Namely, the frequency of handovers may be increased with the introduction of relay nodesRNs, in particular when being implemented with X2 interface functionality. Further, the multi-hop nature of the connection between DeNB and UE (via RN) already makes the handover delay larger than in non-relay based systems and, if X2 handover is not supported (i.e. only S1 handover), then the handover requirements of LTE-Advanced might not be met.
Apart from facilitating handovers, the X2 interface could also be used for other purposes such as LB and ICIC in relaying contexts as outlined above. Similar to the case of UE EPS bearers described above, for a relay architecture of the first category, the X2 interface between the RN and its peer nodes is transparent for the DeNB (unless the peer entity is the DeNB itself). For a relay architecture of the second category, the DeNB is aware of the X2 connection between the RN and its peers.
In view of the above, the applicability of X2 interfaces encompassed those between eNBs in LTE/LTE-Advanced contexts and those between RN and DeNB, eNB and/or other RNs in relaying contexts. Accordingly, a “X2 peer” may be any node having a X2 interface/connection, whether this node is an eNB or an RN.
FIG. 2 shows a schematic diagram of a deployment scenario of a relay-enhanced access network, such as e.g. a Long Term Evolution (LTE) RAN with radio-relayed extensions, supporting X2 interfaces. For the sake of clarity, only the X2 connections of RNa are shown. As shown in FIG. 2, a relay node RN has two types of X2 interfaces or connections, one towards its DeNB and another one to its neighbors via its DeNB. For example the relay node RNa in a first cell has X2 interfaces or connections to its associated DeNB1 in the same cell, as well as to the two relay nodes RNb and RNc in another cell (via their associated DeNB2) and to DeNB2 (via DeNB1). Further, there may also exist one or more other base stations not acting as donor base stations for any one of the relay nodes. As shown in FIG. 2, such other base station may be eNB3 having an X2 interface or connection to DeNB1.
Using the X2 interface in relaying contexts, where there are direct X2 connections between a RN and the DeNB, all its neighboring RNs and non-donor eNBs, in the same manner as in LTE/LTE-Advanced contexts (e.g. of release 8) can have the following negative impacts in terms of resource utilization, signaling load, or the like.
Basically, the X2 information exchanged between the RN and its X2 peers (e.g. DeNB, eNB and other RNs) will be transmitted over the Un interface between the RN and its DeNB, thus consuming expensive radio resources (unlike the X2 interface in LTE release 8, which is operating mostly over wired interfaces between the eNBs).
Since X2 connections between peer entities are independent from each other, the same information have to be sent over the Un interface unnecessarily, i.e. multiple times, when the RN is sending X2 information, such as ICIC messages, towards its peers.
FIG. 3 shows a schematic diagram of a deployment scenario of a relay-enhanced access network, such as e.g. a Long Term Evolution (LTE) RAN with radio-relayed extensions, illustrating X2 signaling messaging, such as e.g. for load balancing purposes. As shown in FIG. 3, RNa is sending the same signaling information towards DeNB1, RNb and RNc, which is indicated by solid, dashed and dotted arrows. After receipt at DeNB1, those X2 messages being dedicated to RNb and RNc are forwarded to DeNB2, and then to the respective RNs. Thus, two of the X2 messages sent over the Un interface (i.e. the X2 interface between RNa and DeNB1) are redundant, as well as one of the X2 messages on the X2 interface between the two DeNBs. The non-donor eNB3 is not involved in such signaling for load balancing purposes according to the present example scenario, but may evidently be involved in a different scenario.
For example, if load balancing is performed between two X2-peer relay nodes in different cells, without involving their donor base stations DeNBs (from a logical point of view), a situation might arise where an already loaded DeNB will become overloaded. This is because a first relay node might be lightly loaded while its donor base station DeNB is overloaded, and the X2 peer of a second relay node might try to transfer the load to the first relay node, which can exacerbate the overload situation of the DeNB as the load of a relay node is indirectly shared by its DeNB through the allocation of resources over the Un interface. While this issue may be addressed by setting handover thresholds also taking into account the load experienced by the backhaul link in the donor cell, such approach may not properly address the issue that the same information is unnecessarily sent many times over the X2 interface.
For example, if interference coordination is performed between two X2-peer relay nodes in different cells, without involving their donor base stations DeNBs (from a logical point of view), a situation might arise where irrelevant information is signaled. This is because a first relay node might transmit interference information relating to its resources towards a second relay node, although the second relay node is assigned such resources which may not even interfere with those of the first relay node e.g. because of being orthogonal to each other. Stated in other terms, a particular relay node may receive X2 messages from each of its neighbors, whether or not these messages are relevant to it. This leads to the issue that the irrelevant information is unnecessarily sent, potentially even many times, over the X2 interface.
For example, if a relay node deployment of (very) high density is assumed, needed SCTP (Stream Control Transmission protocol) associations can be quite high, and as such there can be a significant overhead in maintaining these SCTP associations.
For example, if a relay node is mobile, then whenever it changes its donor base station, it might have to establish X2 connections not only with the new donor base station, but also with all the neighboring non-donor base stations of neighboring cells as well as the new relay node neighbors it discovers, and this might lead to considerable delay and overhead on the Un interface.
In summary, an adoption of X2 signaling communication according to current specifications in a relay-enhanced access network is inefficient in terms of resource utilization because it leads to the transmission of redundant as well as irrelevant messages between neighboring nodes.
In view thereof, there has been proposed that a donor base station in LTE shall support proxy functionality and act as a caching point for X2 messages of its subordinate relay nodes. Namely, when the DeNB receives a X2 message request for the information of its relay node (e.g. resource status information or interference information) by a neighboring non-donor base station eNB, the DeNB could be responsible for answering the request (instead of its relay node). Only when the DeNB does not have the information about its RN which is requested, it will send a new request to the RN. Correspondingly, if a RN requests for information of its neighboring eNB or neighboring RN, it could send a request to its DeNB and then the DeNB will be responsible for acknowledging the RN with corresponding information, either by forwarding the request or by answering the request directly from its cache (thus saving resources).
Yet, this approach addresses the redundancy issues only partially, and it does not at all address the relevancy issues of the X2 messages. That is, all neighbors of the RN still receive the same information, which is a waste of resources over the Un link, because at least part of the information might not be relevant for some of the neighbors. For example, in case of a resources partitioning pattern where the RN is using one third of the total resource blocks in its own cell, then it would make no sense to forward information towards it about the other two thirds of the total resource blocks. The present approach may, however, not prevent such waste of resources due to irrelevancy.
It is to be noted that, while the above uses the X2 interface as a non-limiting example for a (logical) signaling interface, any kind of signaling interface may likewise be applicable, as long as it exhibits comparable features and characteristics as the X2 interface.
Accordingly, there does not exist any feasible solution for facilitating efficient optimization of signaling in relay-enhanced access networks.